1. Field of the Invention
The invention is directed to spectral optical imaging methods and, more specifically, to optical imaging techniques for detecting human cancer in prostate and other tissues.
2. Description of the Related Art
Cancer is a disease that is characterized by uncontrolled cellular growth, whereby cancer cells continue to grow and divide in an abnormal manner. A tumor, defined as any abnormal growth of cells, may be classified as benign or malignant. A benign tumor remains confined or localized to a given site, whereas a malignant tumor is capable of invading other tissues or organs. Most cancers fall into one of three main groups: carcinomas, sarcomas, and leukemias/lymphomas. Of these groups, the most frequently-occurring cancers are carcinomas. Carcinomas may develop from cells that cover the surface of the body, cells of the internal organs, and glandular cells. Glandular cells are found, for example, in the breast and the prostate. Sarcomas are cancers of connective tissue, such as muscle and bone. Leukemias are cancers of the blood forming cells and cells of the immune system.
All cells consist of two major parts: a nucleus and a cytoplasm. The nucleus is the cell's manager. It contains the cell's genetic material in the form of strands of deoxyribonucleic acid (DNA). The cytoplasm, a fluid within the cell, contains proteins, carbohydrates, lipids, and nucleic acids in a water-based solution. A change or mutation in the expression of genes causes cancer to occur. In molecular terms, cancer is a genetic change that occurs within the cell. Two distinct classes of cancer-related genes have been identified: oncogenes and tumor suppressor genes.
Lung cancer, rectum cancer, breast cancer, prostate cancer, urinary cancer, oral cancer, brain cancer and skin cancer represent some of the most frequently occurring cancers. For men, the most common type of cancer is cancer of the prostate. The risk of prostate cancer increases with age. Accordingly, early detection of cancer plays a vital role in reducing mortality from prostate cancer. Present-day screening methods for prostate cancer include digital rectal examinations and prostate specific antigen (PSA) blood tests. There are several different grades or stages of cancer, and these may be ranked using a well-known scale that classifies cancerous and precancerous regions into any of five Gleason Grades, denoted as Stages 1, 2, 3, 4 and 5. Precancerous stages (denoted as stages 1 and 2) correspond to the early stages of cancer.
In an attempt to develop less invasive diagnostic procedures, recent efforts have been directed towards utilization of near-infrared (NIR) optical spectroscopy for cancer and pre cancer detection. NIR techniques, based upon an understanding of cancer at the molecular level, represent an important step toward early detection of cancer. The optical spectrum of a tissue sample contains information about the biochemical composition of that tissue. A primary objective of NIR is to distinguish molecular bonding within cancerous tissue from molecular bonding within normal tissue by detecting fluorescence and Raman spectra from native molecular markers. A gene that is responsible for prostate cancer is attached or tagged with a certain chromophore (molecular marker), such as dye or semiconductor quantum dots, to enhance contrast and resolution in the NIR optical spectroscopy imaging process. The use of molecular markers could enable the imaging process to penetrate more deeply into tissue under examination, thereby enabling doctors and other diagnostic personnel to obtain more information.
State-of-the-art of present techniques for detection of prostate cancer provide limited contrast, low resolution images that do not enable an accurate identification of cancerous tissue. For this reason, the digital rectal examination (DRE), ultrasound imaging, and prostate specific antigen (PSA) blood test are currently the most commonly utilized methods for early detection of prostate cancer. Although X-rays, ultrasound, and magnetic resonance have also been used to detect tumors, these techniques have limited detection capabilities and/or create safety concerns. For example, X-rays are not well-suited for the detection of tumors less than 1 mm in size and, moreover, represent a safety hazard to the patient.
Optical spectroscopy techniques including fluorescence, Raman scattering and light scattering have been used to investigate normal, benign, precancerous and malignant tissues. For example, NIR spectral polarization imaging has been used to image foreign objects dyed with Indocyanine Green at different depths inside prostate tissues. Some disadvantages of fluorescence and Raman scattering methods are a) a point-by-point evaluation cannot be performed; b) a weak diagnostic signal is provided, relative to the amount of elastic scattering that occurs; and (c) direct contact with cancerous tissue must occur in order to make a diagnosis. Elastic scattering detection examines melanin and hemoglobin absorption by focusing on the ultraviolet (UV) and visible regions of light. In these spectral regions, light is highly scattered, making it difficult to detect any microstructure changes that may occur in a tissue sample.
For the sake of computational expediency, a simplification known as the “diffusion approximation” has been widely utilized for describing light propagation in biological media, especially when scattering dominates absorption and the radiant energy fluence rate close to the source is not known. Transport theory is based upon a radiative transfer equation. The solution of this transfer equation in a highly absorbing medium, such as water, surrounded by the non-absorbing tissue, can be simplified and described by the Beer-Lambert law. Note that water absorption is stronger than scattering at specific wavelengths. The attenuation due to absorption is proportional to the concentration (C) of chromophores in tissues, such as water molecules or a specific dye. The optical path length (d) is described by:
                                                        I              =                                                                    I                    0                                    ⁡                                      (                                          1                      -                      R                                        )                                                  ⁢                                                                  ⁢                                  ⅇ                                      -                    acd                                                                                                                                                      or                                                                                                        A              =                                                ln                  ⁢                                                                          ⁢                                                                                    I                        0                                            ⁡                                              (                                                  1                          -                          R                                                )                                                              I                                                  =                acd                                                                        (        1        )            where A is the attenuation measured in optical densities, l0 is the light intensity incident on the medium, l is the light intensity transmitted through the medium, a is the specific extinction coefficient of the absorbing compound in micromolars per cm, c is the concentration of the absorbing compound in micromolars, and d is the distance between the points where the light enters and leaves the medium (sample thickness). The product (ac) is known as the absorption coefficient (μa) of the medium. R is the specular reflection coefficient (Fresnel reflection) from the surface of the sample. When adding absorbing molecules to a host turbid medium (such as tissue), the backscattered or transmitted signal from the sample (water/chromophore-tissue) will be less, especially when absorption dominates.
To calculate the absorption coefficient of a tissue sample, the transmittance (T) or optical density (O. D., T=I/I0(1−R)=10−O.D) of a thin specimen (such as prostate tissue) can be measured in the ballistic region. In a very thin specimen where multiple scattering is negligible, such that d≦ls(ls is the scattering length), or where absorption is much stronger than scattering, the measured absorption coefficient can be obtained from:
                                          μ            a                    =                                    1              d                        ⁢                                                  ⁢                          ln              ⁡                              (                                  1                  T                                )                                                    ,                              where            ⁢                                                  ⁢            T                    =                                    I                                                I                  0                                ⁡                                  (                                      1                    -                    R                                    )                                                      .                                              (        2        )            In relatively thicker tissues, the total attenuation coefficient of a ballistic layer (μt=μs+μa) is measured.
Pursuant to Fresnel's laws of reflection, specular reflection of incident light from a surface is a function of polarization, incident angle, and index of refraction. In the case of unpolarized light, the reflected radiance from a surface is written as
                              R          ⁡                      (                          θ              i                        )                          =                              1            2                    ⁡                      [                                          R                II                2                            +                              R                ⊥                2                                      ]                                              (        3        )            where θi is the incident angle, RII is the reflected electric field parallel to the plane of incidence, and R⊥is the reflected electric field perpendicular to the plane of incidence. For normal incidence (θi=0), equation (3) becomes
                              R          ⁡                      (            0            )                          =                              (                                                            n                  i                                -                                  n                  t                                                                              n                  i                                +                                  n                  t                                                      )                    2                                    (        4        )            where ni is the index of the incident medium, and nt is the index of the transmitted medium.
A linearly polarized light incident on tissue loses its polarization as it traverses the medium for an order of transport length ltr, where
            l      tr        =                  l        s                    (                  1          -          g                )              ,and g is an anisotropy factor. A small portion of the incident light is backscattered by epithelial cells, such that the backscattered light retains its polarization in this single scattering event. The remaining light diffuses into the underlying tissue and is depolarized by multiple scattering. The degree of polarization is defined as:D=(I|−I⊥)/(I|+I⊥)  (5)where the I∥ and I⊥ are the intensities for the parallel and perpendicular components of the reflected or scattered light from the object, respectively.
The contrast is the difference in light intensity in an object or image, and defined as:C=(Imax−Imin)/(Imax+Imin)  (6)where the Imax and Imin are the maximum and minimum intensities of light recorded from the object, respectively.
Scattering and absorption of tissue is caused by the presence of a cellular nucleus (˜10 μm), nuclei (˜3 μm), mitochondria (length ˜1 μm), blood cells, glogi (complicated shapes), cytoplasm, and other tissue structures. The size of the scatterer and the incident wavelength determine the type of scattering that will occur. Also, the distribution of the scatterer size is an important factor in evaluating scattering intensity versus angle
      (          θ      ∼              λ        a              )    .The optical parameters of tissues, such as refractive index n, scattering coefficient μs, and absorption coefficient μa, are responsible for the degree of light scattering in tissue.